Anisotropic conductive films have been widely used in the electronic industry as a connector for testing semiconductor devices and circuit boards, a connector of circuits between boards, a material for mounting a semiconductor device on a circuit board and the like. A known anisotropic conductive film is formed by dispersing conductive particles in a film made from an adhesive insulating resin. However, this anisotropic conductive film is subject to restriction because a fine pitch connection is difficult to achieve and a convex terminal (e.g., bump contact) is required as a connection terminal of a semiconductor element.
To solve this problem, the Applicant proposed, in WO98/07216 etc., an anisotropic conductive film having plural conductive paths insulated from each other and penetrating an insulating film substrate in the thickness direction of the film substrate. The proposed anisotropic conductive film contains plural conductive paths with both ends exposed on the surface of the film substrate made from an insulating resin, and, of these plural conductive paths, those located at the positions allowing contact with the termini of an object to be electrically conducted afford electrical continuity with this object.
However, a close study of the physical properties and the connection state of the connection mate of the anisotropic conductive film proposed above has revealed that the conductive path (metal conductor) in the film has a density higher than necessary, making the film hard to deform, which in turn tends to lower the follow-up property of the film to the connection target (particularly in the case of testing connectors, the degraded follow-up property of the film to the test target sometimes necessitates hard pressing of the film with a high pressure to bring a conductive path in contact with a terminal (electrode) of the test target), and that the density of the conductive path (metal conductor), which is higher than necessary, makes the amount of the insulating resin insufficient to provide an adhesive property when used as a material for mounting, thereby preventing sufficiently high adhesion to an object to be connected.
The above-mentioned conventional anisotropic conductive film is produced by winding plural insulated wires (metal conductor wires having a coating layer made from an insulating resin) around a core member to give a multi-layer roll with the insulated wires densely packed both in the longitudinal direction and the transverse direction, adhering coating layers to make the densely packed insulated wires inseparable, and slicing each insulated wire along the plane forming an angle with the wire section to give a film having a conductive path made of the metal conductor wires. By making thicker the coating layer of the insulated wire to be wound around the core member, the interval of the metal conductor wires (conductive paths) can be widened, which in turn lowers the density of the conductive paths in the film to some degree. While the coating layer can be made thick by repeat coating the metal conductor wires with an insulating resin, the cost necessary for this step is not small at all and the step is impractical. In addition, it is not that the thickness of the coating layer can be increased to any desired level, and the interval of the metal conductor wires (conductive paths) cannot be widened sufficiently. On the other hand, a comparatively large clearance may be formed between adjacent insulated wires when bundling the plural insulated wires and the coating layer of the insulated wires may be melted to widen the interval of the metal conductors. In this case, however, unnecessary voids are formed between the metal conductor wires in the film, thus lowering the strength of the film to the extent that it is not practicable.